Process for producing pure magnesium oxide

ABSTRACT

A process for the preparation of pure magnesium oxide, especially suitable for the preparation of refractory products, from magnesium silicate and magnesium hydrosilicate materials such as olivine, serpentine, garnierite, or the like, the starting material being decomposed (digested) with hydrochloric acid, the residues being separated from the decomposition slurry and the sesquioxides to be regarded as contaminants and other contaminants being precipitated from the crude brine thus obtained by the addition of crude serpentine as a pH increasing substance, the precipitates being separated and the magnesium chloride solution thus obtained being subjected to thermal decomposition, particularly by spray calcination, whereby magnesium oxide is obtained and hydrogen chloride is recovered.

The invention relates to a process for the preparation of pure magnesiumoxide, especially suitable for the preparation of refractory products,from magnesium silicate and magnesium hydrosilicate materials such asolivine, serpentine, garnierite, or the like, the starting materialbeing decomposed with hydrochloric acid, the residue being separatedfrom the decomposition slurry and the sesquioxides to be regarded ascontaminants, and other contaminants, being precipitated from the crudebrine thus obtained by the addition of a pH-increasing substance, theprecipitates being separated and the magnesium chloride solution thusobtained being subjected to thermal decomposition, particularly byspray-calcining, whereby magnesium oxide is obtained and hydrogenchloride is recovered.

The following are suitable as starting materials for this process:so-called ultra-mafitic rocks of the peridotite, dunite, harzburgitegroup and the like, the transformation products thereof, usually knownas serpentine or serpentinite, weathered products of said rocks,halfproducts and byproducts arising during the processing thereof, andindustrial byproducts and waste products of similar composition, as longas they have an MgO content of at least 15% by weight. These materialsare called hereinafter with the term "serpentine" or "crude serpentine".

In known processes of this kind the pH-increasing substance used toprecipitate sesquioxides is generally magnesium oxide, mostly in theform of caustic calcined magnesia. However, the use of such aprecipitant has disadvantages. The serpentines, and similar startingmaterials, usually have a high content of iron and other sesquioxidecontaminants and large additions of precipitant are needed. Since allkinds of caustic magnesia are costly, for the reason that they can beprepared only with a considerable expenditure especially of thermalenergy, the use of caustic magnesia as a precipitating agent imposes aheavy economic burden. If pure magnesium oxide obtained as the endproduct of the process is used as precipitating agent, depending uponthe amount of iron and other sesquioxide contaminants contained in thecrude serpentine or the like starting material, up to about 30% byweight of the obtained magnesium oxide must be circulated internally,and this is economically unacceptable.

It is known from U.S. Pat. 2,398,493 to add calcined serpentine to amagnesium chloride slurry produced by decomposition of serpentine withhydrochloric acid as precipitating agent for contaminants. However, thismethod has the disadvantage that the silicic acid portion of theserpentine is converted, by the calcination, into a form which is moreeasily dissolved or suspended in the magnesium chloride slurry. Theresult thereof is that during the subsequent separation of the residuesor precipitates a larger proportion of silicic acid remains in themagnesium chloride brine and the SiO₂ content in the end product istherefore higher. In addition this increased amount of amorphous ordissolved silicic acid in the magnesium chloride slurry means that theFe(OH)₃ and other hydroxide precipitates obtained during subsequentprecipitation are slimy and therefore difficult to filter. Finally, aconsiderable amount of energy is also required to calcine serpentine.

It is the object of the invention to provide a process of the typementioned at the beginning hereof in which the disadvantages mentionedare eliminated, which process permits economical operation, and whichmakes it possible to obtain easily filtered hydroxy precipitates.

The process according to the invention is characterized in that finelyground crude serpentine having a grain size of between 0 and 1mm,preferably between 0 and 0, 1mm, is used as the pH-increasing substancefor precipitating oxides of iron and of aluminum.

Said precipitating agent ensures complete precipitation of aluminum andextensive precipitation of trivalent iron in the pH range below 5.

If bivalent iron is present in the starting material and if it also isto be removed, according to a special examide of an embodiment of theprocess according to the invention for transforming the bivalent ironinto precipitable trivalent iron, it is possible to introduce air and/orsome other oxidizing agent, such as chlorine or hydrogen peroxide H₂ O₂,into the crude brine under intensive stirring of the latter. If bivalentmanganese is present in the starting material and should be removed, itmay be transformed into quadrivalent manganese by oxidation. This may becarried out in the course of an oxidizing treatment carried out for thepurpose of transforming bivalent iron into trivalent iron; it is alsopossible to carry out an oxidizing treatment additionally to theoxidizing treatment of iron in order to oxidize manganese.

The precipitation occuring in the process according to the invention maybe carried out continuously or intermittently.

One of the advantages of the process according to the invention is thatthe starting material need merely be broken up; no other pre-treatment,especially calcining is needed.

Details of the precipitation procedure are as follows: after the crudeserpentine has been added to the crude brine obtained by separating theundissolved decomposition residue, the pH value of the crude brineinitially increases rapidly to about 4, whereby oxidizing of the stillbivalent iron in the acid crude brine into the trivalent form of iron isencouraged; this, in turn, brings about relatively rapid precipitationof contaminants at the beginning of the whole procedure. Subsequently,as a result of oxidizing Fe⁺⁺ to Fe⁺⁺⁺, the pH value falls slightly andthen rises again. This increase in pH value is steady and slow, the pHvalue remaining below 5.

It is desirable for the crude serpentine used as precipitating agent tobe added to the crude brine in an amount between 1 1/2 and 21/2 timesthe stoichiometrical requirement. In this connection, thestoichiometrical requirement is regarded as the amount of crudeserpentine the MgO content of which is sufficient to neutralize theacidity of the crude brine; the acidity of the crude brine is determinedby the amount of free HC1 present in the acid crude brine and by theamount of crude brine in the oxidic contaminants accessible toprecipitation. In this connection, both the oxidic contaminants whichwere present in the crude brine before the addition of the crudeserpentine, acting as precipitating agent and the oxidic contaminantswhich were introduced into the crude brine with the crude serpentineacting as the precipitating agent, must be taken into account.

It is also advantageous to maintain the temperature of the crude brineat above 80° C for the precipitation of contaminants effected by theaddition of crude serpentine to the crude brine.

Another embodiment of the process according to the invention ischaracterized in that, upon completion of the precipitation ofcontaminants effected by the addition of crude serpentine to the crudebrine, any iron, manganese, nickel still present in the brine, and anyother sesquioxide contaminants remaining in the brine, are precipitatedin known manner by the addition of pH-increasing substances known forthat purpose, preferably caustic magnesia or flue dust arising duringthe calcination of magnesite.

A variant of this embodiment is characterized in that the precipitationcarried out be adding crude serpentine to the crude brine at a pH valueof below 5 is continued only until most of the iron and aluminumhydroxide have been precipitated. Thereafter, the precipitationprocedure is rapidly completed by increasing the pH value to between 6and 7 means of a small amount of caustic magnesia. In this connection,the precipitation effected, for example, by the addition of crudeserpentine, may last for about four hours, while the followingprecipitation, in which the pH value is increased by the addition ofcaustic magnesia, may last for another two hours, for example.

The invention will now be explained in greater detail in the followingexamples.

EXAMPLE 1

1730kg of crude serpentine containing 728kg of MgO, 122kg of Fe₂ O₃,4.66kg of NiO and 5kg of CaO were decomposed with 7330kg of hydrochloricacid containing 1330kg of Cl-. About 710kg of undissolved residue wasseparated from the decomposition slurry and 8350kg of acid crude brinewas thus obtained. This crude brine containing 713kg of MgO, 113kg ofFe₂ O₃, 4.6kg of NiO, 1330kg of Cl- and 1.8kg of CaO, was mixed with647kg of crude serpentine with a grain size of 0 to 0.1mm as theprecipitating agent and air was introduced, under intensive stirring,into the crude brine, the temperature of which was maintained at above80° C. The pH value increased to about 4 and precipitation was completedin a few hours. The precipitated material together with the undissolvedportion of the crude serpentine added as the precipitating agent wasseparated resulting in a filter cake weighing 823kg. The filter cakecontained 225kg of MgO, 157kg of Fe₂ O₃, 1kg of NiO and 1.4kg of CaO.The remaining pure brine contained 748kg of MgO, 0.08kg of Fe₂ O₃ ,5.15kg of NiO, 1324kg of Cl and 3kg of CaO.

EXAMPLE 2

1585kg of crude serpentine containing 667kg of MgO, 112kg of Fe₂ O₃,4.27kg of NiO and 4.59kg of CaO was decomposed with 6746kg ofhydrochloric acid containing 1220kg of Cl-. About 625kg of undissolvedresidue was separated from the decomposition slurry and 7706kg of acidcrude brine was thus obtained. This crude brine containing 643kg of MgO,104kg of Fe₂ O₃, 4.20kg of NiO, 1220kg of Cl- and 1.8kg of CaO was mixedwith 597kg of crude serpentine with a grain size of 0 to 0.1mm as theprecipitating agent and air was introduced, under intensive stirring,into the crude brine, the temperature of which was maintained at above80°C. The pH value increased to about 4 and after 4 hours of reactionwas quickly increased to pH 6.8 by the addition of 98kg of flue dust.The flue dust contained 82kg of MgO, 5.20kg of Fe₂ O₃ and 2.60kg of CaO.This allowed the precipitation to be completed within 1 hour. Theprecipitated material was separated together with the undissolvedportions of crude serpentine and caustic magnesia, resulting in a filtercake weighing 830kg and containing 255kg of MgO, 145kg of Fe₂ O₃, 1.85kgof CaO and 4.27kg of NiO. The remaining pure brine (7571kg) contained721kg of MgO, <0.01kg of Fe₂ O₃, 4.25kg of CaO, <0.01kg of NiO and1210kg of Cl-.

We claim:
 1. A process for the preparation of pure magnesium oxide fromstarting materials selected from the group consisting of magnesiumsilicate and magnesium hydrosilicate materials comprising dissolving thestarting material in hydrochloric acid to form a slurry comprising crudebrine and residues, separating the residues from the slurry, addingfinely ground crude serpentine having a grain size of between 0 and 1mmto the brine as the pH increasing substance for precipitating oxides ofiron and aluminum, separating the precipitates and thermally decomposingthe magnesium chloride solution thus obtained to obtain magnesium oxideand hydrogen chloride.
 2. A process according to claim 1, whereinbivalent iron present in the starting material is converted intoprecipitable trivalent iron by the introduction of air and/or anotheroxidizing agent into the crude brine under intensive stirring.
 3. Aprocess according to either claim 1 or 2, wherein the crude brine isheld at a temperature above 80° C. during precipitation of contaminantsaffected by the addition of crude serpentine to the crude brine.
 4. Aprocess according to either claim 1 or 2, wherein the crude serpentineused as precipitating agent is added to the crude brine in an amountbetween 11/2and 21/2times the stiochiometric requirement.
 5. A processaccording to either claim 1 or 2, wherein upon completion of theprecipitation of contaminants affected by the addition of crudeserpentine to the crude brine, any iron, manganese, nickel or othersesquioxide contaminants remaining in the brine are precipitated in aknown manner by the addition of pH increasing substances known for thatpurpose.
 6. A process according to either claim 1 or 2, wherein theprecipitation carried out by the addition of crude serpentine to crudebrine at a pH value of below 5, is continued only until most of the ironand aluminum hydroxide has been precipitated, the precipitation processthen being rapidly completed by increasing the pH value to between 6 and7 by means of caustic magnesia.
 7. A process according to either claim 1or 2, wherein said finely ground crude serpentine has a grain size ofbetween 0 and 0.1mm.
 8. A process according to claim 5, wherein the pHincreasing substance added to the crude brine to precipitate iron,manganese, nickel or other sesquioxide contaminants remaining in thebrine is selected from the group consisting of caustic magnesia or fluedust resulting from calcining magnesia.
 9. A process according to eitherclaim 1 or 2, wherein the crude serpentine used as precipitating agentis added to the crude brine in an amount between 11/2and 21/2times thestoichiometric requirement, and wherein, upon completion of theprecipitation of contaminants affected by the addition of crudeserpentine to the crude brine, any iron, manganese, nickel or othersesquioxide contaminants remaining in the brine are precipitated in aknown manner by the addition of pH increasing substances known for thatpurpose.
 10. A process according to either claim 1 or 2, wherein thecrude serpentine used as precipitating agent is added to the crude brinein an amount between 11/2and 21/2times the stoichiometric requirement,and wherein the precipitation carried out by the addition of crudeserpentine to crude brine at a pH value of below 5 is continued onlyuntil most of the iron and aluminum hydroxide has been precipitated, theprecipitation process then being rapidly completed by increasing the pHvalue to between 6 and 7 by means of caustic magnesia.
 11. A process forthe preparation of refractory product grade pure magnesium oxide fromstarting materials selected from the group consisting of magnesiumsilicate and magnesium hydrosilicate materials comprising dissolving thestarting material in hydrochloric acid to form a slurry comprising crudebrine and residues, separating the residues from the slurry, addingfinely ground crude serpentine having a grain size of between 0 and 1mmto the brine as the pH increasing substance for precipitating oxides ofiron and aluminum, separating the precipitates and thermally decomposingthe magnesium chloride solution thus obtained to obtain said puremagnesium oxide and hydrogen chloride.